Luminescent phosphor screens are used in conjunction with CCD detectors (or other silicon detectors) as high resolution, high dynamic range x-ray imagers in x-ray crystallography, medical and industrial imaging. The performance of these devices is heavily dependent on the characteristics of the phosphor material used. Desirable characteristics for a phosphor of this type include high photoluminescent efficiency, peak emission in the red or near-infrared wavelength range, and low afterglow. The desire for high photoluminescent efficiency is due to the obvious advantage of higher sensitivity. Peak emission in the red or near-infrared bands enables the phosphor output to match the maximum quantum efficiency of the silicon detectors. Low afterglow is important for high dynamic range imaging (high afterglow levels lead to ghost images and streaking).
Recently, phosphors based on ZnSe.sub.1-x Te.sub.x and ZnSe.sub.1-x Te.sub.x :Cu:Cl have been developed. These phosphors show high x-ray luminescence with a peak emission near 650-700 nm (depending on the exact doping composition used), and have an energy efficiency of about 20%. Furthermore, they show low afterglow levels as compared to previously available x-ray phosphors. These phosphors are described in detail in V. Valdna, et al., "ZnSe.sub.1-x Te.sub.x solid solutions," Joumal of Crystal Growth, Vol. 161, 1996, pp. 177-180.
The phosphors disclosed by Valdna have many of the necessary qualities of a good imaging phosphor. However, it has been found that, in their basic formulation, phosphors of this type suffer from a nonlinear output in that, during the initial luminescence of the phosphor while being exposed to constant x-ray flux, the luminescence of the phosphors increases over a finite period of time before stabilizing. Moreover, the phosphors exhibit an undesirable amount of afterglow once the x-ray exposure has been discontinued.